spriter is a bunch of combined parts. they can have a very small file sizes, were as the png files can add up when all animations are added together(run, jump etc).
For png files, think 11 frames (for the slowest fluid movement) per animation. Then say each png is about .5 - 1 Mb well you get the difference can add up (6 -11 Mb per anim) if each animation needs to be loaded into memory for each level.
For spriter you just need to load the parts only. This means you can make the initial image larger and still save memory cause only they are loaded. the Anim is really just a set of cords and rotations, like any bone animation for 3d games.
it very cool what you two have done:)